As airplane architecture is increasingly dependent upon electrical systems to operate the aircraft, the size and criticality of the primary electrical generation systems also increases. As an example, the Boeing 787 employs two engines to drive four variable frequency starter/generators (VFSGs). In this arrangement, two starter/generators are mounted to each engine gearbox. Both of the generators do operate at the same speed and frequency, since they are directly geared from the same engine shaft, but the AC power output of the two generators is not synchronized in phase. Therefore, the power generated by the two generators cannot be tied directly together. In order to protect each generator, the distribution buses and related equipment connected to each generator must be physically separated. Additional bus tie breakers must also be installed to power each bus while the normal generator is not available. Such physical limitations increase the complexity, size, weight, and cost of the power distribution system.
Connecting two or more generators in parallel to a common bus could reduce this complexity and enable improved load balancing, but requires precise synchronization of both generators. If the generators are not precisely synchronized, large currents known as “circular currents” will be present, causing a loss in efficiency, excessive heat dissipation, and the potential to damage the generators. Typical six-pole generators should be mechanically aligned within approximately one degree. Known methods, such as indexing gears and keying shafts, could be applied, but these methods alone would not provide sufficient tolerances to allow the generators to be paralleled.
Additionally, many former airplane designs employed a Constant Speed Drive (CSD) mechanism to produce fixed 400 Hz power from a variable engine shaft speed. CSD mechanisms have been used, notably on the Boeing 747-400 airliner, to synchronize the main generators so that paralleled buses are feasible. However, CSD systems are heavy, expensive, and unreliable, and have been dropped in favor of directly geared variable frequency generators.
In designing new generations of aircraft, reducing the volume and weight of the power systems is critical. The four-bus architecture currently used in aircraft power distribution systems consumes an excessive amount of the equipment bay.
Therefore, what is needed is the ability to reduce the physical barriers and number of bus breakers required to operate the electrical systems of an aircraft, by configure the four generators that are required to power the aircraft in a two-bus architecture. Such an arrangement simplifies the electrical panels, reduces the amount of electrical equipment that is required, and relaxes the separation requirements internal to the panels as well as for wiring routed throughout the aircraft.